1. Technical Field
The present invention relates to a secondary battery, and more particularly, to a cap assembly of the secondary battery.
2. Description of the Related Art
A secondary battery is a rechargeable battery that can be made into a small size or a high power. Common types of secondary batteries include the nickel-hydrogen battery, the lithium battery and the lithium-ion battery. The lithium-ion secondary battery can have a various shape, for example, square and cylindrical batteries.
For the lithium-ion battery, when it is excessively charged or its positive electrode and negative electrode are short-circuited, the electrolyte with lithium salt and organic solvent is electrolyzed at the positive electrode and the lithium metal is extracted at the negative electrode, and thereby the deterioration of the battery characteristics and the short-circuit occur. In particular, excessive charge generates excessive current, which damages an apparatus using the battery as a power source. Moreover, excessive charge generates gas to increase the internal pressure, which may cause explosion and ignition of the battery.
In an effort to overcome these difficulties, the lithium-ion battery generally has a safety device thereinside, such as a shut-down separator, a PTC device to cutoff current when the temperature increases, and a safety vent to discharge the pressure when the internal pressure increases.
For example, U.S. Pat. No. 5,985,478 entitled Pressure responsive cap assembly for rectangular battery to Kim, and issued on Nov. 16, 1999, discloses a safety device in which a through-hole formed on the negative electrode is simply riveted by use of a rivet connected to a tab while an insulator and a gasket are disposed over and below the negative electrode.
However, the above structure has a limit to increasing sealing capacity, and in particular, since the secondary battery generates a large amount of gas from the inside, it has a problem that the sealing capacity of the through-hole decreases relatively.
Japanese Patent Laid-open No. 11-219692 discloses a thin metal plate to seal a hole of a cap plate. However, this structure has a problem in that there occurs a crack in a safety vent when the external physical impact is loaded.
Moreover, if the secondary battery used as a power source for an electronic device generates excessive current, the excessive current is transmitted to the electronic device to thereby damage the electronic device. Especially, such problems are more serious to the battery for a motor driven device which repeats the charge and discharge within a short time.
Accordingly, there is a need for a safety device which can prevent the explosion and cutoff excessive current when it occurs.